The effect of protein levels on growth, postprandial excretion and tryptic activity of juvenile mullet Mugil platanus (Gu¨nther)

Abstract

The objective of the present work was to determine the optimum dietary protein level for juvenile mullets. Five isocaloric diets were formulated to contain increasing levels (300,350,400,450 and 500 g kg 1) of crude protein (CP) corresponding to 18.7MJ metabolizable energy kg 1. All diets were tested in triplicate. Each experimental unit was composed of a 50 L tankwith 50 juveniles (mean SE initial weight and length equal to 1.17 0.02 g and 4.34 0.03 cm respectively). Diets were o¡ered ¢ve times a day until apparent satiation for 35 days. No signi¢cant di¡erence(P40.05) was observed in survival rate, feed e⁄ciency and body composition between treatments. However, weight gain, feed consumption and speci¢c growth rate were higher in ¢sh fed the 350 g kg 1CP level than those fed the highest protein content diet(500 g kg 1 CP). The amount of postprandial ammonia excreted by mullet was linearly related to protein intake. Intestinal tryptic activity was inversely proportional to the percentage of dietary CP. It is likely that diets containingo350 g kg 1CP will be needed for on-growing mullet, especially when reared in ponds with abundant natural food.